Plastic and reconstructive surgery often entails the localization and clinical evaluation of a flap of skin and subcutaneous tissue which is supplied by isolated perforator vessels and that is potentially suitable for grafting in another part of the body. Perforators pass from their source vessel to the skin surface, either through or between deep muscular tissues. Well-vascularised flaps are good candidates for grafts.
For example, abdominal donor-site flaps have become the standard for autologous breast reconstruction since the early 1980s. Within the abdomen, free fat options range from complete transverse rectus abdominis musculocutaneous (TRAM) flaps to isolated perforator flaps, such as the deep inferior epigastric artery (DIEA) perforator flap. Perforator flaps have allowed the transfer of the patient's own skin and fat in a reliable manner also in other areas of tissue reconstruction, with minimal donor-site morbidity. Flaps that relied on a random pattern blood supply were soon supplanted by pedicled, axial patterned flaps that could reliably transfer great amounts of tissue. The advent of free tissue transfer allowed an even greater range of possibilities to appropriately match donor and recipient sites. The increased use of perforator flaps has escalated the need for a pre-operative familiarity of an individual's particular anatomical feature of the DIEA and its perforating branches, particularly given the significant variation in that anatomy of the vascular supply to the abdominal wall.
Localization and evaluation of perforators is a painstaking and time-consuming process. Pre-operative computed tomography angiographic (CTA) imaging is often performed to do the localization. Such an approach entails considerable expense and has the additional complication that the surgeon must mentally correlate the images from the previously acquired 3D modality with the current 2D view of the patient now lying on the operating table. The search for a more favorable imaging modality is thus continuing, with recent interest in the use of indocyanine green (ICG) fluorescence imaging, wherein blood circulation is assessed through the skin on the basis of a fluorescence signal. Fluorescence in ICG with an emission peak around 830 nm occurs as a result of excitation by radiation in the near-infrared spectral range. Excitation light with a wavelength around 800 nm can be produced, for example, by a diode laser, light emitting diodes (LED), or other conventional illumination sources, such as arc lamps, halogen lamps with a suitable bandpass filter. The skin is transparent to this wavelength.
ICG strongly binds to blood proteins and has previously been used for cardiac output measurement, hepatic function evaluation, and ophthalmic angiography, with few adverse reactions. Evaluation of ICG fluorescence signals can be used to locate perforators. Since the skin surface near a perforator generally accumulates more blood and at a faster rate than the surrounding tissue, once ICG is injected, perforators tend to fluoresce brighter and faster than the surrounding tissue. This rapid, high-intensity fluorescence enables visual localization of the perforator. Often, however, the surgeon is interested not merely in localization but also in evaluation and comparison to support good clinical decision making. The surgeon needs to decide which of several perforators the best graft candidates are. Here, simple visual observation while fluorescence rapidly accumulates and dissipates does not suffice. For example, the tendency of residual ICG from successive injections to accumulate in tissue and to gradually raise the background brightness with each injection further confounds easy visual discrimination of the best candidate perforators. In addition, ICG sometimes moves exceedingly slowly over several minutes making such on-the-fly analysis very challenging and subjective. A surgeon will make an assessment by raising the following questions:
1) How much ICG-bound blood is in the tissue?
2) How long does it stay in the tissue?
3) How quickly does it move through the tissue?
4) After the bolus is injected, in which order do anatomical areas light up?
These questions are difficult to answer on a subjective basis. Accordingly, there is a need for more advanced image processing and display methods to apply objective standards to localize and evaluate perforators.